1. Field of the Invention
The present invention relates to an improvement in devices for calibrating high frequency pressure transducers that are used for the measurement of rocket motor combustion instability or other transient phenomena. The invention is particularly useful for calibrating transducers for use in applications where the pressure medium is corrosive or thermally severe, and, as a consequence, the pressure transducer requires a thermal or corrosion protection system.
2. Description of the Prior Art
The gases in a burning rocket motor offer a severe thermal environment in which high frequency pressure transducers for measuring combustion instability or other transient phenomena require protection from melting by recessed mounting and/or thermal coatings such as silicon rubber. Such recessed mounting and protective coatings have an undesired effect, however, in that they alter the response of the transducer to pressure fluctuations in the rocket combustion chamber proper. The error in the altered response may vary from fractions of a percent to orders of magnitude. Although various calibration schemes have been proposed in the prior art, such schemes, in general, have proved to be cumbersome and expensive and have been little used. For the most part, errors due to altered response have been recognized and simply tolerated.
The prior art calibration techniques have involved the use of either "shock" devices or "driven" devices. Although mechanical shock devices have been used, pneumatic shock tubes have been more common. A pneumatic shock tube usually consists of a pressure chamber fitted with a rupture diaphragm. When the diaphragm is ruptured, a shock wave forms that is impacted on the mounted, protected transducer. The transducer may or may not be pressurized to an elevated mean pressure. Shock tubes are generally not readily portable, being either heavy or long, or both. Shock tubes require partial disassembly and replacement of diaphragms and further require expenditure of an appreciable amount of compressed gas for each test.
"Driven" devices based on mechanical vibration of known masses upon a fluid column have been used, but generally have been limited to frequencies below 1 or 2 kHz. Driven acoustic devices have been developed by the National Aeronautics and Space Administration (NASA) and by the United States Air Force.
The NASA development is described in publications of R. E. Robinson entitled: "Improvement of a Large Amplitude Sinusoidal Pressure Generator for Dynamic Calibration of Pressure Transducers," NASA CR-120874, Feb. 1972, and "Dynamic Response of High Frequency Pressure Transducers to Large Amplitude Sinusoidal Pressure Oscillations, " NASA CR-2000, Apr. 1972.
In the NASA device, termed a sinusoidal pressure generator, reference and test pressure transducers attached to an acoustic chamber are stimulated by means of an interrupted stream of air as produced by a perforated rotating disc. Fairly smooth sinusoids produced at low frequencies give way to distorted sinusoids at higher frequencies (3 kHz) that gradually subside into the acoustic noise. In its intended usage, the NASA device suffers a limited frequency range, low portability and moderate compressed gas consumption.
The Air Force device, developed in the late 1970's, consists of a tunable resonant cavity excited by a vibrating membrane pump driven by a piezoelectric disc. The unit combines moderate portability with good high frequency response (20 kHz), but mean pressure capability is limited by the strength of the membrane. In its intended usage, the Air Force device produces very sinusoidal pressure fluctuations, in consequence of which many calibration measurements at different manually-adjusted response frequencies are required in order to describe the frequency response of a transducer and its protection system.